$B_s^0 \to J/\psi \phi$ - ATLAS

The study of the decay \Bst is of significant interest to particle physics as it allows the measurement of the \Bs mixing phase. The \BtoKns channel provides a valuable testing ground for measurements of \Bst due to its equivalent topology and similar helicity structure of the final states, with the advatage of higher statistics. \ilumi \ of ATLAS $pp$ collision data at a center-of-mass energy of 7 TeV are used to reconstruct \Bd and \Bs mesons. A total of \Nsigdsta \Bd signal decays are observed after subtracting the background, with a fitted mass of \Bdmasssta. For \Bs signal \Nsigssta decays are observed, with a fitted mass of \Bsmasssta. Within the statistical uncertainties, the fitted masses of both $B$ mesons are consistent with the world average values.